The present invention relates to a digital code decoding apparatus and, more particularly, to a digital decoding apparatus which is applicable as well to the case where a received waveform is extremely distorted owing to characteristics of a transmission system, for instance, where a so-called eye pattern of the received waveform is not fully open.
FIG. 1 shows an example of a conventional decoding apparatus. For example, in the case of transmitting digital information shown in FIG. 2A, through use of Manchester codes, "0s" in data of the information to be transmitted are each converted to a "high-to-low" form and "1s" to a "low-to-high" form; and such a waveform as shown in FIG. 2B is transmitted. This waveform is transmitted over a line or like transmission system and is distorted by its transmission characteristics to a gently varying waveform as depicted in FIG. 2C. The distorted waveform is received and applied to an input terminal 10, from which it is provided to an equalizer 11 for correction of the transmission distortion of the received waveform. The corrected waveform is compared, by a comparator 12, with a reference voltage V.sub.R from a reference voltage source 13 and shaped into a binary waveform which has high and low levels as depicted in FIG. 2D. The thus shaped waveform is provided to a clock regenerating section 14 and a sampling section 15. Based on changing points in the output waveform from the comparator 12, the clock regenerating section 14 produces at its output a sampling clock which has a frequency that is twice higher than the transmission bit rate and lags by 90.degree. in phase with respect to the changing points as shown in FIG. 2E. The sampling section 15 samples the output waveform from the comparator 12 with a 1/2 frequency-divided clock of the regenerated clock, regenerating the original transmitted waveform as shown in FIG. 2F. The clock regenerating section 14 usually has an arrangement in which a pulse signal created by applying the received signal to a differentiation circuit is provided to a phase locked loop circuit to obtain a clock synchronized with the changing points in the comparator output waveform and the phase of the thus obtained clock is delayed 180.degree. (i.e., 90.degree. behind the transmitted bit), thereby regenerating the sampling clock. The sampling section 15 can be implemented by a D flip-flop. The sampled output waveform from the sampling section 15 is provided to a decoding section 16, which yields a "0" or "1" depending upon whether the input waveform is "high-to-low" or "low-to-high". In this way, data of the same information as the original transmitted information is produced as shown in FIG. 2G.
In general, in the cases where the transmission distance is long and the transmission line includes bridge taps, the transmission characteristic does not have a flat frequency characteristic and the transmitted waveform is subject to distortion as depicted in FIG. 2C. The transmitted information bits "0" and "1" are "high-to-low" and "low-to-high" in the Manchester code; and therefore when the transmission distortion is not too large, what is called an eye pattern, shown in FIG. 3A, is obtained by superimposing respective bits of the received waveform for the same period of time. Where such an open eye pattern is obtainable, it is possible to correctly determine whether each of the received waveform is "high-to-low" or "low-to-high", by suitably selecting the sample points, i.e. timings and the reference voltage V.sub.R for comparison with the received signal level in the comparator 12, as indicated by crosses in FIG. 3A. When the transmission distortion is very large, however, the eye pattern does not open as shown in FIG. 3B, and the transmitted information cannot be reproduced correctly. Where the distortion by the transmission system is large, as mentioned above, it is customary in the prior art to employ a method in which the transmission characteristic is compensated for by the equalizer 11 for opening the eye pattern, as shown in FIG. 3A. The equalizer is a filter which has a characteristic inverse from the frequency characteristic of the transmission system, and this filter can be implemented by both analog and digital filters.
The equalizer employing an analog filter, for correcting the transmission characteristic, is formed by one or more LC filters or RC active filters.
The equalizer employing a digital filter also corrects the transmission characteristic by a filter whose characteristic is inverse therefrom, as is the case with the analog filter, but the method of implementing the filter differs from that in the case of the analog filter. FIG. 4 shows an example of the arrangement of the digital filter. A waveform applied to the input terminal 10 is sampled by an A/D converter 11a and thereby digitized, and the digital output is applied to a plurality of cascade-connected delay circuits 11b, in each of which it is delayed by the sampling interval. The output of the A/D converter 11a and the output of each delay circuit 11b are provided to a multiplier 11d, wherein they are multiplied by a value stored in a coefficient register 11c, and the respective multiplied outputs are added together in an adder 11e, the added output of which is converted by a D/A converter 11f into analog form. This filter performs processing in the time domain. A filter of a desired characteristic can be constituted by properly setting the number of stages of the delay circuits 11b, the coefficient registers 11c and the multipliers 11d, and the values of filter coefficients to be set in the registers 11c.
In the conventional decoding apparatus provided with the equalizer, shown in FIG. 1, it is necessary to determine the coefficient of each stage of the filter depicted in FIG. 4 such that complex transmission characteristics may be corrected. To determine the filter coefficients, a predetermined bit pattern (hereinafter referred to as a training pattern) is transmitted from the transmitting side, for example, prior to the transmission of information, and the decoding apparatus of the receiving side determines the filter coefficients so that a correct bit pattern may be decoded from the received signal. In such training of the decoding apparatus in accordance with the transmission characteristic, the conventional equalizer calls for the transmission of training data of more than 1K-bit or so. This will seriously impair the efficiency of transmitting a short packet a bus-type LAN or the like, for example. The reason for this is that since the transmission characteristic varies with the position on the bus where a terminal of each transmitting side is connected thereto, the reception of information must be preceded by training of the receiving characteristic for each of the different transmitting sides. Another reason is that the information to be transmitted is as short as dozens to hundreds of bits in almost all cases. Accordingly, the transmission of 1K-bit or more data for training is undesirable in terms of the utilization efficiency of the bus.
For the correction of complicated transmission characteristics, the number of stages of the filter shown in FIG. 4 inevitably increases, besides each stage of the filter needs to use the multiplier 11d involving a large amount of hardware. Accordingly, the amount of hardware necessary for the entire filter is so large that its economical construction is difficult. Furthermore, the use of the multiplier 11d constitutes an obstacle to high-speed processing, making high-speed transmission difficult.
In the case where the characteristic of the transmission line is poor and the received waveform is seriously deteriorated as mentioned above, the conventional method for compensating for the characteristic of the transmission line through use of an equalizer possesses the defects of poor transmission efficiency, difficulty in high-speed transmission and an increase in the amount of hardware of the equalizer which introduces difficulty in economical construction of the decoder.